Is it permissible to give as charity the value of the crayons and brush given to us by the kindergarten teacher, knowing that they might belong to other people and that it is difficult to return them?

It is not permissible to accuse a teacher of theft without proof, and the fundamental principle is that a teacher is a role model. If the pens were a gift from the teacher to the students, there is no obligation to return them, because a gift is valid through mutual exchange, especially if custom dictates it and the pens are not of significant value. However, if you wish to be cautious, you may ask the teacher about them if possible, but this is not obligatory as long as you are certain that she would not take them if you were to return them.